1. Field of the Invention
The present invention pertains generally to measuring and test devices and more particularly to manufacturing defect analyzers for testing digital components.
2. Description of the Background
Devices for testing printed circuit boards have typically fallen into three major categories, i.e., functional testers, in-circuit testers and manufacturing defect analyzers. Each of these three major categories of testers is briefly described below.
Functional testers comprise devices which have typically measured the output performance of a printed circuit board based on a series of input signals. Although functional testers provide an excellent way of testing the end desired performance of a particular board, they are usually incapable of determining the location of defects in the board, or components mounted on the board. As boards become larger, more complex, and costly, the chances of a defect occurring on a board, due to the many traces on the board, or one of the many components mounted thereon, increase greatly. Since functional tests merely indicate whether or not the board is operating properly and do not provide an indication of the location of a defect, the usefulness of functional testers as an overall test system is limited.
Because of the limitations of the functional testers in locating defects in boards, in-circuit test devices were developed. In-circuit test devices actually test individual components mounted on the board and test conductivity paths through the traces of the board by attaching a series of connector pins to various nodes on the circuit so that test signals can be applied to the components to determine operability. Algorithms have been developed to carefully limit the power which can be applied to components to prevent damage. Digital backdriving techniques have been integrated into the in-circuit testing scheme to allow both analog and digital components to be effectively tested in the in-circuit test devices. As circuit topologies have become more complex and the demand for faster in-circuit testers has grown, the in-circuit testing devices have become more and more expensive. This has resulted in a need for a less expensive device capable of testing simpler defects, such as open traces on a board or bent pins on a component.
Manufacturing defects analyzers were originally developed as inexpensive test devices for testing simple defects such as open circuits or short circuits on the prined circuit board. Since a high percentage of the faults on a board are produced by solder shorts between traces and other simple defects, manufacturing defect analyzers were developed to provide an inexpensive device for detecting a majority of these faults. Hence, manufacturing defects analyzers were able to fill a need for detecting simple defects on a printed circuit board without employing expensive in-circuit testing devices. Although analog testing techniques have been employed in some manufacturing defect analyzers, to date, very few have incorporated digital testing techniques because of the complexity and expense involved. Hence, manufacturing defect analyzers primarily remain as simple fault detection testers. Test Systems, Incorporated, located in Tempe, Arizona has designed several techniques for testing digital components in a simple and easy manner which can be employed in manufacturing defect analyzers. The Test System's circuit utilizes a voltage potential between the V.sub.CC node and the signal node which is sufficient to forward bias a single diode junction but insufficient to forward bias two diode junctions. If the IC is inserted in reversed orientation, a large amount of current will flow from the VCC node to the input/output node (signal node) indicating that the IC has been inserted in reverse orientation. However, parallel conductive paths must be carefully guarded to ensure that false readings are not obtained from the test system. The guarding techniques required by the Test Systems, Incorporated device can often times be somewhat complex and difficult to implement. Hence, a need exists for a simple method of unambiguously determining whether an IC has been placed in the circuit in proper orientation.